The invention relates to a reduction objective, a projection exposure apparatus, and method of use thereof for exposing patterned information onto a reduced image plane for use in lithography applications, such as integrated circuit fabrication.
The desirability of creating lithography systems which operate with incident light wavelengths below 193 nm to achieve imaging structures or patterns with resolution below 130 nm has been established. In fact, there has been a need for lithography systems using soft X-ray, or so called extreme ultraviolet (EUV), wavelength incident light, such as xcex=11 nm or xcex=13 nm light, to obtain image resolution in the below 100 nm range. The resolution of a lithography system is described by the following equation:
RES=k1xc2x7/NA
where k1 is a specific parameter of the lithography process, xcex is the wavelength of the incident light and NA is the numerical aperture of the system on the image side. This equation can be used to establish design criteria for soft X-ray lithography systems. Thus, for a numerical aperture of 0.10, the imaging of 100 rim structures with 13 nm radiation requires a process with k1=0.77. Alternatively, with k1=0.64, which occurs with 11 nm radiation, imaging of 70 nm structures is possible with a numerical aperture of 0.10.
For lithography imaging systems in the soft X-ray region, multilayer coated reflective systems, such as Distributed Bragg Reflectors (DBRs) and mirrors, are used as optical components. These multilayer systems employ alternating layers of different indices of refraction. The particular multilayer reflective system depends on the wavelength of incident light used. For example, with incident light of xcex=11 nm alternating layers of Mo/Be are preferred and with incident light of xcex=13 nm Mo/Si layers are used. In the case of projection objectives used in soft X-ray or EUV microlithography, the reflectivity of these multilayer systems is approximately 70%. It is, therefore, desirable to use as few optical components in the projection objective as possible to ensure sufficient light intensity at the image. With high intensity light, four-mirror projection objective systems were found to be useful in correcting imaging errors at NA=0.10.
Other requirements for soft X-ray projection objectives, in addition to using a limited number of multilayer reflectors, relate to obscuration, image field curvature, distortion, telecentricity on both the image and the object side, free working distance, and aperture stop positioning and accessibility. Obscurations, for example the central obscuration apparent in Schwarzschild systems, create intolerable degradation in image quality. If an obscuration-free light path is required, then, in the case of centered systems, an off-axis image field must be used. In order to provide image formats of 26xc3x9734 mm2 or 26xc3x9752 mm2, it is advantageous to design the projection objective system as an annular field scanner. In such systems, the useful secant length of the scanning slit should be at least 26 mm, and the annular width should lie in the range of 0.5 mm to 2 mm in order to make uniform illumination and illumination-control and dose-control possible.
Regarding distortion, a distinction between static and dynamic or scan distortion is made. Scan distortion is the effective distortion which is obtained by integration of the static distortion over the scanning path. The limits for magnification-corrected, static distortion follow essentially from the specifications for contrast and CD variation.
Image-side telecentricity is also desired in soft X-ray lithography systems. Whether telecentricity is possible on the object side depends on the type of projection application used. If the projection system uses a reflection mask, then a telecentric optical path on the object side is not possible. If transmission masks, for example stencil masks, are used then a telecentric optical path on the object side can be realized.
In order to enable clean limitations of the beam, the aperture stop should be physically accessible. The image-side telecentricity requirement, referred to above, means that the entrance pupil of the last mirror would lie in or near its focal point. In order to obtain a compact design and maintain an accessible aperture stop, it is recommended that a beam-limiting element be placed before the last mirror. This, preferably, results in the place of the beam-limiting element at the third mirror.
Four-mirror projection or reduction objectives have become known from the following publications:
U.S. Pat No. 5,315,629
EP 480,617
U.S. Pat. No. 5,063,586
EP 422,853
Donald W. Sweeney, Russ Hudyma, Henry N. Chapman, David Shafer, EUV optical Design for a 100 mm CD Imaging System, 23rd International Symposium of microlithography, SPIE, Santa Clara, Calif., Feb. 22-27, 1998, SPIE Vol. 3331, p. 2ff.
In U.S. Pat. No. 5,315,629, a four-mirror projection objective with NA=0.1, 4xc3x9731.25xc3x970.5 mm2 is claimed. The mirror sequence is concave, convex, concave, concave. From EP 480,617, two NA=0.1, 5xc3x97, 25xc3x972 mm2 systems have become known. The mirror sequence is concave, convex, arbitrary/convex, concave.
The systems according to U.S. Pat. No. 5,063,586 and EP 422,853 have a rectangular image field, for example, at least 5xc3x975 mm2, and use a mirror sequence of convex, concave, convex, concave. These generally decentered systems exhibit very high distortion values. Therefore, the objectives can only be used in steppers with distortion correction on the reticle. However, the high level of distortion makes such objectives impractical in the structural resolution regions discussed here (xe2x89xa6130 nm).
From U.S. Pat. No. 5,153,898, overall arbitrary three to five-multilayer mirror systems have become known. However, the disclosed embodiments all describe three-mirror systems with a rectangular field and small numerical aperture (NA less than 0.04). Therefore, the systems described therein can only image structures above 0.25 xcexcm in length.
Furthermore, reference is made to T. Jewell: xe2x80x9cOptical system design issues in development of projection camera for EUV lithographyxe2x80x9d, Proc. SPIE 2437 (1995) and the citations given there, the entire disclosure of which is incorporated by reference.
In the known systems according to EP 480,617 as well as U.S. Pat. No. 5,315,629 and according to Sweeney, cited above, it was found to be disadvantageous that the outside-axially used part of the primary mirror conflicts with the wafer-side sensor structures of a projection exposure installation when not very large free mechanical working distances greater than 100 mm are realized. By using mirror segments which are placed xe2x80x9cnear the image fieldxe2x80x9d, these conflicts occur only at significantly lower distances (≈10 mm).
Six mirror projection or reduction objectives with all multilayer mirrors in centered arrangement to an optical axis have become known, e.g., from U.S. Pat. No. 6,033,079. The system according to U.S. Pat. No. 6,033,079 is telecentric on the image side. To provide small reflection angles at the object side, i.e., at the reticle, a long distance between the primary multilayer mirror and the reticle according to U.S. Pat. No. 6,033,079 is chosen. A disadvantage of this reduction objective is the extension, or lengthening, of the projection objective in the direction of the optical axis.
Thus, it is desired to provide a projection objective which is suitable for lithography with short wavelengths (at least below 193 nm and, preferably below 100 nm) which does not have the disadvantages of the prior art mentioned above, uses as few optical elements as possible and, yet, has a sufficiently large aperture and fulfills the telecentricity requirements as well as all other requirements for a projection system operating with incident light of wavelengths below 193 nm.
According to an aspect of the invention, the shortcomings of the prior art are overcome by using a reduction objective that includes four-mirrors (primary, secondary, tertiary, and quaternary). Using this four-mirror system, high transmission efficiency is achieved at wavelengths in the soft x-ray and EUV region with a multilayer mirror system of 70% reflectivity and an aperture in the range of NA≈0.10. The four-mirror objective according to the invention is thus characterized by high resolution, low manufacturing costs and high throughput. In another embodiment a system using six multilayer mirrors is provided.
In both systems a first set of multilayer mirrors is in centered arrangement with respect to a first optical axis and a second set of multilayer mirrors is in centered arrangement with respect to a second optical axis. At least one additional mirror is disposed at grazing incidence between said first and said second set of multilayer mirrors, wherein said additional mirror defines an angle between said first optical axis and said second optical axis.
In a preferred embodiment of the invention, it is provided that an aperture stop lies on or near a mirror, especially on the tertiary mirror. In this embodiment, the aperture stop is physically accessible and the reduction objective is compact and free from vignetting.
In another embodiment of the reduction objective of the invention, the secondary mirror and the quaternary mirror are concave. In still a further embodiment of a four-mirror-system, the four mirrors are arranged in the convex-concave-convex-concave sequence.
The asphericities discussed herein refer to the peak-to-peak or peak-to-valley (PV) deviation A of the aspherical surfaces in comparison to the best fitting sphere in the used region of the mirrors. In the embodiments of the invention discussed herein, these are approximated by a sphere, the center of which lies on the figure-axis of the mirror and the meridional plane of the aspherical element intersects in the upper and lower and point of the used region. The data on the angles of incidence, as provided below in Table II, refer to the angle between the particular incident radiation and the normal to the surface at the point of incidence. The largest angle of incidence of a ray is established by a rim-ray at one of the mirrors.
Preferably, the optically free working distance on the wafer side is 60 mm and the free working distance on the reticle side is at least 100 mm.
As will be appreciated from this disclosure by persons of ordinary skill in the art, the objectives described above can be used not only for soft X-ray lithography, but for other wavelengths, both in the EUV range and outside of this range, without deviation from the invention. Nonetheless, the invention is preferably used at soft X-ray and EUV wavelengths in the region of 193 nm and below. Wavelengths within this range can be generated using excimer lasers.
In order to achieve a diffraction-limited system, it is preferred that the design part of the rms wave front section of the system is at most 0.07xcex. It is further preferred that the reduction objective is designed so that it is telecentric on the image side. In systems with a transmission mask, the projection objective is also designed to be telecentric on the object side. In projection systems which are operated with a reflection mask, a telecentric beam path without illumination over a beam splitter which reduces transmission greatly, such as is shown in JP-A-95/28 31 16, is not possible on the object side. Therefore, the chief ray angle in the reticle is chosen so that shading-free illumination, that is no obscuration, is possible. Overall, the telecentricity error on the wafer should not exceed 10 mrad, and preferably is in the 5 mrad to below 2 mrad range. This ensures that the change in the image scale or distortion lies within tolerable limits in the depth of field.
In a preferred embodiment of the invention, the first set of multilayer mirrors comprises four mirrors and the second set of multilayer mirrors comprises two mirrors. The grazing-incidence mirror is arranged in such a way that the first and the second optical axis are parallel to each other. Such an embodiment provides for a short projection objective in axial direction and for small reflection angles at the object side.
According to another aspect of the invention, the reduction objective is used with a mask and a soft X-ray or EUV exposure source, for example, in a projection exposure apparatus, such as those used in lithography for integrated circuit fabrication. In the embodiment, the projection exposure apparatus has a reflection mask and, in an alternative embodiment, it has a transmission mask.
The projection exposure apparatus is preferably designed as an annular-field scanner illuminating an off-axis annular field. 10Advantageously, it is provided that the secant length of the scanning slit is at least 26 mm and that the annular width is greater than 0.5 mm, so that uniform illumination is made possible.
Another advantage of the objective of the invention, is that the asphericity of the aspherical optical elements is small enough so that the system requirements of being xe2x80x9cdiffraction-limitedxe2x80x9d and having high reflectivity multilayer mirrors can be achieved, such that the resulting accuracy requirements on these surfaces in all spatial frequency regions from the free diameter of the mirror to atomic dimensions can be obeyed.
According to another aspect of the invention, a method of integrated circuit fabrication using a projection exposure apparatus including a reduction objective is provided. The method comprises the steps of providing a mask, providing a soft X-ray or EUV, illumination source, and providing four mirrors (primary, secondary, tertiary, and quaternary) in centered arrangement with respect to an optical axis, wherein the primary mirror is a convex mirror and the secondary mirror has a positive angular magnification.
Traditionally, approximately 40 pairs of alternating Mo/Si DBR layers are used to create mirrors with high reflectivity of 13 nm wavelength incident light. Similarly, high reflectivity of 11 nm wavelength radiation, requires approximately 70 pairs of Mo/Be alternating layers. By using large numbers of DBR layers, the acceptance angle of mirrors and thus their objective systems is only a few degrees and decreases with increasing angle of incidence. Furthermore, with this increasing angle of incidence, detrimental phase effects can occur. If the test-point-related mean angle of incidence varies greatly over a system surface, then layer packets with changing thickness must be applied. These disadvantages of existing multilayer mirror objective systems are reduced in the present invention in which the objectives have a low mean angle of incidence and a low surface-specific variation around the mean angle of incidence.